In projection displays using liquid crystal on silicon (LCoS) panels, there are usually opportunities for improving performance using compensating retarders. These compensators can be used to remove residual in-plane retardance of the panel in the black state, and/or for removing light leakage due to the finite field of view of the liquid crystal layer. Moreover, compensators can nullify leakage associated with geometrical rotation from McNeille polarizing beamsplitters (PBS) in finite f-number systems. They can also compensate for polarization distortion induced by wire grid polarizers (WGP) in certain systems, particularly those that are off-telecentric. In an actual system, two or three of these coupled effects can be compensated using a compound element that resides in the space between the PBS and the LCoS panel.
Such compound elements have been shown to accomplish the desired compensation via computer modeling, but practical demonstrations frequently fall significantly short of theoretical predictions. In practice, this is the result of subtle compensator deficiencies that often determine the final system contrast.
Stretched polymer retarder films are frequently used, for instance, in circular polarizers to reduce glare in direct view displays. Early retarder films were manufactured using polyvinyl alcohol (PVA)—a substrate mass-manufactured to produce polarizing films. But PVA substrates are thin and hygroscopic, thus possibly necessitating the additional lamination of a support substrate with a moisture barrier function, such as cellulose acetate butyrate. Much of the PVA based product available today is not suitable for contemporary retarder film applications, due to excessive thickness and poor uniformity.
Much of the more recent development work in manufacturing retarder films was for standard twisted nematic (STN) panel compensation and for field-of-view compensation in direct view active matrix LCD (AMLCD) displays. Polycarbonate has emerged as the substrate of choice for LCD applications. In direct-view LCD displays, stretched polycarbonate films are attached to the LCD polarizers using pressure sensitive adhesive (PSA).
In principle, retarder films for projection can also be manufactured using polycarbonate materials that are similar to those used for direct-view LCD compensator applications. The performance and reliability requirements of projection compensators rule out many of the commercially available retarder film products. Compensator retarder films for projection would optimally balance properties such as high optical clarity, low defect density, low refractive index (1.52) for index matching to glass to minimize isotropic reflections, low birefringence to minimize anisotropic reflections, uniform casting or extrusion to minimize transmitted wavefront distortion and birefringence texture, uniformity of optic axis and retardance, low surface energy for adhesive bonding, high rigidity to resist non-uniformities resulting from mechanical strain, and performance stability. These properties should be maintained despite the temperature cycling and high luminance that the materials face in the projection environment.
Polycarbonate materials have a relatively high refractive index (1.59), but they have been manufactured to otherwise meet many of the above challenges posed in the projection environment. The properties of polycarbonate materials are nonetheless significantly challenged under the high-heat, high-luminance conditions of the projection environment.
One issue related to using polymer retarder films in projection systems relates to thermal gradients that develop under the intense illumination conditions. Nonuniform heating in a typical float glass compensator can induce stress birefringence that is directly observable in a crossed polarizer light box where it is manifested as, for example, a corner light leakage. In some instances, the temperature gradients are sufficient in the LCoS panel port of a projector that dark state uniformity is not adequate. To minimize this effect, glass with low photoelastic coefficient is selected or the overall thickness of glass is minimized. But even in such systems, mechanical strain can develop between the glass substrate and the optical bonding/sealant layers.
To further reduce this strain, low durometer elastomeric sealants with adhesive properties have been used to bond the retarder film layer to the glass substrate. Such elastomeric adhesives mechanically isolate the retarder film from the glass substrate. But when more rigid sealants are used with typical polycarbonate, the temperature window of operation is too narrow for projection systems.
Typical pressure sensitive adhesives that are frequently used for mounting polarizer to glass in high temperature polysilicon (HTPS) projection systems degrade under the intense illumination of higher brightness systems, such as an LCoS projector. Moreover, pressure sensitive adhesives typically have a low refractive index (1.46), giving high reflections that can reduce contrast in LCoS systems. Also, the haze from pressure sensitive adhesives contributes an additional scatter component that can reduce sequential and ANSI contrast.
There are alternatives to pressure sensitive adhesives for mounting retarder film to a rigid substrate. Ultraviolet cure acrylic sealants can be extremely clear and can have an intermediate refractive index. When carefully cured, the strain on the retarder film can be minimized. But such acrylic sealants usually have a poor coefficient of thermal expansion (CTE) match to the substrate, which results in a mechanical load when the temperature deviates from the cure temperature. In applications involving severe temperature cycling, delamination due to the CTE mismatch can ultimately occur.
Thus, improved compensators would be useful, especially in high intensity projection systems.